A plasma processing apparatus may be engaged in any of various types of production processing, e.g., etching, ashing and plasma deposition, executed on a processing target substrate such as a semiconductor wafer. As further miniaturization of semiconductor elements adopting multiple layer structures with greater numbers of layers is pursued, the processing will need to be executed with a minimum of collateral damage. For instance, in processing executed by using radicals, the extent of ion damage must be minimized by actively promoting the reaction induced by the radicals. Namely, various measures are taken to prevent damage caused by excess ions, such as undesirable blending of materials constituting different layers at the wafer, destruction of an oxide film, entry of contaminants and alteration in the characteristics of the substrate. In addition, ion impact, likely to lead to low selectivity, should be avoided as much as possible during an etching process executed by defining the selection ratio with a high level of accuracy.
It is known that ion damage to wafers can be effectively prevented by generating plasma with a potential set as low as possible. A device utilized to generate such plasma may include a coil (helical resonance coil) wound around a reaction container that can be depressurized, so as to create a standing wave by causing the coil to resonate in a full wavelength mode with high-frequency power with a predetermined wavelength supplied thereto and generate plasma from a processing gas by forming an induction field within the reaction container (see, for instance, Japanese Laid Open Patent Publication No. 2007-142444). It is assumed that inductively-coupled plasma can be generated at a node at which the phase-voltage switching potential is 0 by canceling out the phase-voltage with the anti-phase voltage.
However, since the potential of plasma generated via a coil resonating in such a full wavelength mode is substantially 0, no sheath is formed between the plasma and the inner wall of the reaction container. This means that only a very low level of self-cleaning performance for cleaning the inner wall of the reaction container through sputtering or the like is achieved. In addition, if a processing gas with a high reaction product vapor pressure and a high reaction product deposition rate is used, reaction products are bound to readily adhere to the inner wall of the reaction container where the plasma is generated. It is to be noted that while this concern may be addressed by adjusting, for instance, the coil electrical length so as to ensure that the phase-voltage and the anti-phase-voltage do not completely cancel each other out, a change in the coil electrical length will require complicated and time-consuming device adjustment.
An object of the present invention, having been achieved by addressing the issues discussed above, is to provide a plasma processing apparatus and a plasma processing method with which plasma and can be generated while minimizing the damage to the processing target substrate and disallowing ready adhesion of reaction products to the inner wall of the reaction container, by simply controlling the high-frequency power supplied to the coil, i.e., by selecting the optimal high-frequency power to be supplied to the coil.